Effect of Increasing Alumina
The density of clusters of surface micro-crystals in a cobalt glaze increases
with increased alumina.
The design is created by adhering a mask and then applying the main
glaze. After drying,
bowl is ~ 4 inches in diameter.
bowl is ~ 6 inches in diameter
150 deg F an hour to 250 deg F
400 deg F an hour to 1800 deg F
300 deg F an hour to 2050 deg F
120 deg F an hour to 2310 deg F with a hold of 20 minutes at 2310 deg F
300 deg F an hour to 1850 deg F then a 2 hr hold at 1850 deg F
300 deg F an hour to 1750 deg F then a 1 hr hold at 1750 deg F
300 deg F an hour to 1700 deg F then a 3 hr hold at 1700 deg F
25 deg F an hour to 1650 deg F then a one hour hold at 1650 deg F
K2O 0.12
Al2O3 0.50
SiO2 2.8
molecular percent Silica 64.9%
Added:
2.5% Cobalt Carbonate
K2O 0.11
Al2O3 0.54
SiO2 2.8
molecular percent Silica 62.3%
Added:
2.5% Cobalt Carbonate
K2O 0.29
Al2O3 0.76
SiO2 3.61
molecular percent Silica 62.3%
The effect is subtle, in keeping with a modest
increase in alumina.
the mask is removed and the open area filled in with a
second glaze, using a bulb syringe
with a needle applicator.
Image of the higher alumina glaze zincSr_Z3TV_0PK
inside
outside
Image of the lower alumina glaze zincSr_Z3TV_0 with inlay epk_tsp_rutile_Z3V
oxidation firing to cone 10 in an electric kiln
Firing profile
Up Fire profile cone 10
Down Fire Profile cone 10
Clay body is a grolleg porcelain from Tacoma Clay Art Center.
glaze compositions
Lower Alumina Glaze:
Empirical Formula zincSr_Z3TV_0:
Na2O 0.42
CaO 0.03
MgO 0.25
SrO 0.11
ZnO 0.07
Higher Alumina Glaze:
Empirical Formula zincSr_Z3TV_0PK:
Na2O 0.42
CaO 0.02
MgO 0.24
SrO 0.13
ZnO 0.08
Inlay Glaze:
Empirical Formula inlay_epk_tsp_rutile_Z3V:
Na2O 0.67
CaO 0.03
MgO 0.01
P2O5 0.20